The steel wall of a pipeline or tubing may be subject to internal corrosion. This is dependent, obviously, on the matter that is transported through the pipeline or tubing. Both liquids and gases may attack the steel wall.
Two forms of corrosion may be of interest, namely uniform corrosion and pitting. In the case of uniform corrosion the wall thickness decreases more or less uniformly and it will be clear that this could lead to the pipeline becoming unserviceable, because the wall will become too weak to resist the internal pressure. Pitting could lead to leakage and this phenomenon must also be avoided.
It is obviously of great importance to be regularly informed about the state of the pipeline or tubing to be able to take measures in time.
Since downtime is costly, the inspection of pipelines or tubings should preferably be carried out during operation. Moreover, pipelines or tubings are often installed where access is difficult, e.g., underground or under water, so that methods of internal inspection are to be preferred.
It is known that internal pipeline or tubing corrosion may be determined from the internal dimensions of the pipeline or tubing obtained by means of a pig adapted to be carried along by the flow of matter through the pipeline or tubing. Such a pig is fitted with an array of probes protruding outward from the pig and spring-load to press against the inner wall of the pipeline or tubing in a radial direction.
The probes are fitted with displacement pick-ups and the pig is further equipped with apparatus for processing the signals from the displacement pick-ups and for recording the results of such processing.
The pig is further fitted with means for the determination of distance. This may be a time clock, whose signal is regularly recorded and may be used in conjunction with the flow velocity to determine the locations of recordings. It is also possible to use a supporting wheel whose revolutions indicate the location of a recording. In this way the location of uniform corrosion and pitting can be determined quantitatively.
However, such a pig provided with an array or probes is less suitable for use in pipes having a small diameter. Such small pipes are often used, e.g., in heat-exchangers, flow lines, etc.
Further, it is at time sufficient to obtain only qualitative information about the condition of the pipe wall without the need of an expensive instrumented pig and a time-consuming processing of signals.